I would like to know which one of the tww flex transponders (flexdf desfire, flexnt) should become the vivokey?
I have seen that they both fulfil the ISO standard for payment cards; so what exactly is the difference between those two?

Actually payment cards rely on much more than just the ISO 14443 standard… that standard is more about air interface, RF behavior, command set, and the like… it has nothing to do with chip features. An EMV payment card must support cryptographic signatures or at the very least be able to supply signed cryptographic tokens to the payment terminal. It gets complicated because there are several methods of communicating payment data outlined in the numerous EMV standard “books” that are expensive to buy from EMV.

VivoKey will not be either of the flex products we have now. It will be based on a new secure element chip from NXP that can run java card applets, perform cryptography internally (including creating keys and key pairs), and can actually meet the technical requirements for EMV contactless transactions. Whether or not we get banks and processors to the table is another matter.